The present invention relates generally to the field of radiography and, more particularly, systems for generating cardiac angiograms.
Radiologic examination forms an integral and essential part of evaluating cardiac disease. A radiograph or radiogram is a film or other record produced by the action of actinic rays, typically x-rays, on a sensitized surface. The shadow of the heart, as seen on a standard or "plain" radiograph or by fluoroscopy, is useful for evaluating gross disease processes, such as chamber enlargement. Such studies, however, only demonstrate the outer borders of the heart and great vessels. Considerably more information is obtained when the blood is opacified by injecting a contrast medium into the vascular system, so that the inner borders of the cardiac chambers and vessels can be visualized.
Of particular interest to the present invention is the imaging and diagnosis of coronary artery disease, especially that induced by atherosclerosis. Until recently, the diagnosis of coronary artery disease depended primarily on the ability of a physician to interpret the significance of chest pain described by patients. Objective confirmation required the recognition of transient or persistent electrocardiographic changes, which usually indicated the presence of myocardial ischemia, necrosis, or other secondary myocardial change.
Selective cine coronary angiography, a specialized radiographic technique, provides a useful clinical approach to the precise demonstration of morphologic changes in the human coronary artery vasculature. Under local anesthesia, a catheter is percutaneously introduced, typically in the brachial or femoral artery, and passed retrograde directly into the ascending aorta. The catheter tip is then introduced directly into each of the two coronary orifices. Multiple small doses of contrast medium are injected directly into the orifice of each coronary artery, with the patient typically positioned in different projections.
In a conventional coronary angiography study, x-rays are admitted toward a subject while contrast medium passes through the branches of the coronary tree. Some of the x-rays are absorbed or scattered, while others pass through the subject toward an image intensifier. Upon reaching the image intensifier, the passed-through x-rays are converted to form an optical image which, through the use of mirrors and/or beam splitters, may be focused at either or both a film camera (i.e., 35 mm motion-picture or cine camera) and a T.V. or video camera. In particular, the film camera is used for capturing multiple images as individual photographs. The T.V. camera, on the other hand, is used for capturing multiple images as analog or digital video frames (hereinafter collectively referred to as digital images), which may be acquired, stored, processed and combined to form dynamic video image sequences.
For a typical coronary angiographic exam, approximately ten sequences of images of the heart are obtained. Since the heart and its vasculature are three-dimensional in structure, several projections (i.e., different perspectives) are often obtained. A single sequence typically lasts approximately eight seconds and comprises a plurality of frames captured at a frame rate of about 7.5 to 60 frames per second (fps); sometimes, rates as high as 90 fps or 150 fps are used.
Since excess irradiation is undesirable, the x-rays are emitted in a pulsed fashion, at the lowest possible exposure level. Typically, each frame within a sequence is formed by transmitting an x-ray pulse of about 20-100 micro-Roentgens over a period of 2-10 milliseconds. In a technique similar to flash or strobe photography, the cine camera and T.V. camera capture the pulses while operating at slower shutter speeds (i.e., longer durations than the x-ray pulse) so that none of the x-rays are wasted. In contrast to other body tissues, the actively contracting heart requires shorter pulses and more frames to capture a focused, non-blurred image sequence.
It would be desirable to provide a cardiac acquisition system which includes the ability to acquire both cine-film and digital images simultaneously from two planes, e.g., from frontal and lateral perspectives. These "biplane" studies should include studies acquired at 30 fps or more for each plane. To reduce scatter (i.e., stray x-rays from the other plane), each plane should be acquired at a different time (out of phase) relative to the other plane. Furthermore, the system should operate with only a single digital image acquisition processor, thereby reducing costs. The present invention fulfills this and other needs.